Cyclic oxygenation of BOD-containing water

ABSTRACT

BOD-containing water such as sewage is mixed with active biomass and a first quantity of oxygen feed gas in a first cycle for biochemical oxidation to produce oxygenated liquid-solid and unconsumed oxygen-containing gas of lower purity than the feed gas. The unconsumed oxygen is discharged and a second quantity of oxygen feed gas is introduced for mixing in a second biochemical oxygenation cycle.

CROSS-REFERENCES TO RELATED APPLICATION

The following applications relating to oxygenation of BOD-containingwater were filed simultaneously with this application:

Ser. No. 838,467, High Oxygen Utilization in BOD-Containing WaterTreatment, J. R. McWhirter; Ser. No. 838,498, Biochemical Oxidation withLow Sludge Recycle, E. K. Robinson and J. R. McWhirter; Ser. No.838,499, Bio-Oxidation with Low Sludge Yield, J. R. McWhirter; Ser. No.838,500, Staged Oxygenation of BOD-Containing Water, J. R. McWhirter.

BACKGROUND OF THE INVENTION

This invention relates to a method of and apparatus for treatingBOD-containing water by oxygenation. The BOD-containing water may forexample be municipal sewage, chemical waste from petrochemical or paperplants, or fermentation liquor.

With few exceptions, biochemical oxidation methods have employed air asthe oxygen source. The large quantity of air required to supply thenecessary oxygen is largely due to the 4/1 dilution with nitrogen, andtypically only 5-10% of the potential oxygen mass transfer efficiency ofthe method is attained. However, the air is "free" and the large amountof energy supplied to the air is normally sufficient to mix and suspendthe bacterial solids (active biomass) in the liquid.

The direct use of oxygen instead of air in treatment of municipal andchemical wastes has been considered for many years because of itspotential advantages in reducing the quantity of gas required. Moreover,it has been speculated that the rate and completeness of suchbiochemical reactions are suppressed by low dissolved oxygen (DO) levelsin the liquor. Because of the additional cost of oxygen, it must be usedsparingly and effectively. This necessitates a small volumetric ratio ofgas-to-liquor as compared to air aeration. Also, the partial pressure ofoxygen in the aerating gas must be sustained at high level to achieveeconomies in the cost and operation of aeration equipment while stillobtaining high overall levels of oxygen utilization. The prior art hasnot discovered a method which maintains high oxygen partial pressure inaeration while simultaneously utilizing a high percentage of the oxygencontained in the valuable gas. Convention air aeration techniques do notsatisfy these requirements.

Other conventional gas-liquid contacting techniques such as packed orplate-type columns, sparged columns, or agitated gas-liquid columnswhich are commonly employed in chemical processing are not well suitedfor this particular purpose. Although these systems can be designed toachieve a high percentage oxygen absorption, they are not readilyadapted to the handling of mixed liquid-solid suspensions such asencountered in activated sludge processes for waste water treatment.Neither are the conventional systems suited for contacting large volumesof liquor and small volumes of gas with high rates of dissolution andwith low energy consumption.

The achievement of both high oxygen utilization and high oxygen partialpressure in biochemical oxidation processes is further complicated bythe evolution of diluent gases from the mixed liquor undergoingaeration. Usually the BOD-containing feed water to the process isnitrogen-saturated with respect to air. While mass transfer of nitrogenis not a consideration when air aeration is employed, it becomes a verysignificant factor when the nitrogen content of the aeration gas isreduced and the volume of aeration gas becomes small. This is becausethe dissolved nitrogen will be stripped from the liquor into the gas andwill reduce the oxygen partial pressure of the gas. Other gases evolvedfrom the liquor which are inert to the biochemical reaction will have asimilar effect, e.g., argon and moisture. Carbon dioxide, which is aproduct of the oxidation, will also evolve in substantial quantity andfurther suppress the oxygen partial pressure.

If the oxygen-enriched aeration gas is utilized effectively, then itsvolume relative to air will be very low, e.g., 1/90. While this offersopportunities for cost savings in gas compression, it aggravates theproblems of liquid mixing and of oxygen dilution with impurities. Thetotal energy input to the small quantity of gas for purposes of oxygensolution may now be far less than that required for suspending andmixing the solids in the liquid. The inert gases evolved from the liquorwill also impair the oxygen partial pressure to a greater extent as thequantity of aeration gas is reduced.

It is an object of this invention to provide an improved system fortreating BOD-containing water with oxygen gas for biochemical oxidation.

Another object is to provide a system characterized by high rate ofoxygen transfer to the BOD-containing water per unit of energy input,which represents a substantially higher energy transfer efficiency ascompared to conventional atmospheric air aeration techniques.

Still another object is to provide a system for oxygenation ofBOD-containing water characterized by high oxygen partial pressure andhigh oxygen utilization efficiency.

Other objects and advantages of this invention will be apparent from theensuing disclosure and appended claims.

SUMMARY

This invention relates to a method of and apparatus for the treatment ofBOD-containing water by cyclic biochemical oxygenation in contact withbiomass.

The prior art has been unable to quantitatively elucidate the complexmulti-component gas-liquor transfer process and related liquor phasereaction characteristic of oxygen aeration of BOD-containing water. Thisis undoubtedly one reason why oxygen has not been commercially utilizedfor biochemical oxidation of sewage. This combined gas-liquor masstransfer process and liquor phase reaction have now been positivelyidentified. The method and apparatus of the invention effectivelyutilizes the relative component equilibrium solubilities andstoichiometry to afford a highly efficient system characterized by highpercentage oxygen absorption while simultaneously maintaining a highoxygen partial pressure in the aerating gas system.

In one method aspect, as a first oxygenation cycle BOD-containing waterand biomass, i.e. liquor, are mixed with a first feed gas quantitycomprising at least 50% oxygen (by volume) and having oxygen partialpressure of at least 7.3 p.s.i.a. while simultaneously continuouslyrecirculating one of such fluids against the other fluids in a chamberfor at least 10 minutes and with sufficient mixing and gas-liquidcontact energy input to consume at least 60% (by volume) of the oxygenin the first feed gas. First oxygenated liquor or liquid-solid, andfirst unconsumed oxygen-containing gas are formed in this first cycle,the gas comprising 10-70% oxygen but of lower oxygen purity than thefirst feed gas and having oxygen partial pressure of at least 1.47p.s.i.s. Because more oxygen is consumed in this cycle than gas isevolved from the liquor, the product gas quantity from the firstoxygenation cycle is appreciably less than the oxygen feed gas quantity.This first unconsumed oxygen-containing gas is discharged from thechamber at the end of the first cycle, and a second oxygenation cycle isinitiated having the same general parameters.

In the second cycle, a second feed gas quantity comprising at least 50%oxygen and having oxygen partial pressure of at least 7.3 p.s.i.a. ismixed with second BOD-containing water and second biomass in the samechamber while simultaneously continuously recirculating one of thefluids against the other fluids. The second cycle mixing also continuesfor at least 10 minutes and with sufficient mixing and gas-liquidcontact energy to consume at least 60% of the oxygen in the second feedgas to form second oxygenated liquor or liquid-solid, and secondunconsumed oxygen-containing gas comprising 10-70% oxygen but of loweroxygen purity than the second feed gas and having oxygen partialpressure of at least 1.47 p.s.i.a. The second unconsumedoxygen-containing gas is discharged from the chamber at the end of thesecond oxygenation cycle, and the first and second cycles are thereafterrepeated.

The mixing liquor in this second cycle is at least in part composed ofthe first oxygenated liquid-solid formed in the first cycle, andunoxygenated liquor affords the balance of the second BOD-containingwater and biomass. Additional oxygen feed gas may be introduced duringeach cycle as the oxygen is consumed, for example to maintain constantaeration gas pressure. As a further alternative, additionalBOD-containing water and biomass are introduced to the chamber duringthe first and second oxygenation cycles.

In one embodiment, the biomass is concentrated from the oxygenatedliquid-solid, for example in a clarifier, and recycled in sufficientquantity to provide volatile suspended solids content (MLVSS) of atleast 3,000 p.p.m in the first and second oxygenation cycles. In wastewater embodiments the latter adds very little MLVSS as compared to thesludge (active biomass). Accordingly the sludge must have an appreciablyhigher MLVSS value to afford at least 3,000 p.p.m. on dilution with thewaste water. For waste water systems the MLVSS comprises at least 0.55of the total suspended solids (MLSS). By way of example, MLVSS/MLSSratios of 0.70 to 0.75 have been measured in the treatment of wastesfrom two different municipalities.

One apparatus embodiment of the invention includes a liquor storageenclosure, an oxygen gas source, and at least one oxygenation chamberpreferably fixedly positioned within the storage enclosure below theliquor level with its lower end in fluid communication with the storageenclosure. A gas-tight cover is provided over the chamber's upper end.Oxygen supply conduit means extend between the oxygen gas source and theoxygenation chamber, and conduit means are also provided for dischargingunconsumed oxygen-containing gas from the upper portion of theoxygenation chamber. A vent valve is positioned within the gas dischargeconduit. Means are included for mechanically mixing the oxygen gas andliquor in the oxygenation chamber, as for example a motor-drivensurface-type aerator.

Gas flow control means for this apparatus include a gas inlet flowcontrol valve arranged to maintain a predetermined gas pressure in theoxygenation chamber, and an inlet shut-off valve, both in theaforementioned oxygen supply conduit. Means are included for sensing gaspressure in the oxygenation chamber, with signal transmitting means fromthe pressure sensing means to the gas inlet flow control valve. Cyclecontrol means are provided for simultaneously closing the gas inletshutoff valve and opening the gas vent valve to permit venting ofoxygen-depleted aeration gas (i.e. the unconsumed oxygen-containing gas)under the force of the unbalanced hydrostatic head flowing into thechamber lower end from the liquor enclosure. The chamber liquor levelrises and displaces the oxygen-depleted aeration as through thedischarge conduit and vent valve. The cycle control means also performsthe function of thereafter simultaneously closing the gas vent valve andopening the gas inlet shutoff valve, thereby permitting repetition ofoxygen gas inlet flow through the control valve to the oxygenationchamber.

In another apparatus embodiment, an oxygen gas sparger is positionedbelow the liquor level in the oxygenation chamber, and gas-liquormechanical mixing means such as a motor-driven propeller is alsopositioned below the liquor level. A gas blower is provided with thesuction side in flow communication with the oxygenation chamber upperportion and the discharge side in flow communication with the sparger.

The gas flow control means of this apparatus embodiment includes lowliquor level sensing means in the oxygenation chamber and signaltransmitting means from this level sensing means arranged to close theoxygen gas inlet control valve when inflowing oxygen gas has downwardlyforced the liquor level to a predetermined elevation. Means are alsoprovided for sensing the chamber oxygen gas content, and signaltransmitting means to open the chamber vent valve when the sensed oxygengas content descends to a predetermined value. As used herein "gascontent" may be either composition, e.g. oxygen purity, or pressure.High liquor level sensing means are included with signal transmittingmeans therefrom arranged to close the vent valve and open the oxygen gasinlet control valve when the rising liquor reaches a predeterminedelevation.

The method and apparatus of this invention may be used to treatBOD-containing water in a manner significantly more efficient than thewidely used air aeration treatment processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view taken in cross-sectional elevation ofapparatus according to one embodiment of the invention, including afixedly positioned oxygenation chamber and a clarifier;

FIG. 2 is a schematic view taken in cross-sectional elevation of anotherembodiment characterized by a floating oxygenation chamber, with asubmerged propeller and gas sparger as the gas-liquor mixing means;

FIG. 3 is a schematic view taken in cross-sectional elevation of stillanother embodiment characterized by a gas cycle control system utilizinga recirculation blower for exhausting oxygen-depleted gas;

FIG. 4 is a schematic view taken in cross-sectional elevation of amultiple oxygenation chamber embodiment with staging of oxygenatedliquid-solid;

FIG. 5 is a graph showing the gas and liquor compositions in terms ofoxygen, nitrogen and carbon dioxide as a function of cycle time for theoxygenation step of a typical cycle using municipal waste as feed; and

FIG. 6 is a graph showing the variation of instantaneous oxygen feedrate, oxygen transfer rate to the liquor, and the cumulative overallpercentage oxygen absorption during the FIG. 5 cycle.

Referring now to the drawings and in particular FIG. 1, BOD-containingwater as for example sewage is introduced to enclosure 11 throughconduit 12. Active biomass is introduced to enclosure 11 through conduit13, although it may alternately be premixed with the BOD-containingwater and introduced through conduit 12. The biomass is preferablyobtained from oxygenated liquid-solid discharged from enclosure 11 inconduit 14. Solids concentration may be practiced in clarifier 15, wherethe oxygenated liquid-solid is separated into purified water andconcentrated biomass. Clarifier constructions are well-known to thoseskilled in the bio-oxidation art, and may for example include arotatable scraper 16 at the lower end to prevent coning of theconcentrated biomass. The latter is withdrawn from clarifier 15 throughconduit 17 and at least a portion thereof is recycled through pump 18 toconduit 13 for introduction to liquor enclosure 11. The purified wateris discharged from clarifier 15 through conduit 19. In the embodimentwherein waste comprises the BOD-containing feed water, the recycledsolid is commonly referred to as activated sludge.

Oxygenation chamber 20 is fixedly positioned in enclosure 11 below theliquor level by members 21, with gas-tight cover 22 below the edge ofdischarge weir 23. The lower end 24 of oxygenation chamber 20 is influid communication with enclosure 11. The system also requires oxygengas source 25, which may for example be a pressurized gas container or athermally insulated liquid vessel with vaporizing means. Oxygen gassupply conduit 26 communicates between gas source 25 and oxygenationchamber 20. Gas inlet control valve 27 and shut-off valve 28 are bothprovided in supply conduit 26, with valve 28 preferably downstream ofcontrol valve 27. First pressure switch 29 may also be provided inoxygen supply conduit 26 if desired, but is not essential.

The feed gas must comprise at least 50% oxygen so that the gas-liquormixing time (to achieve at least 60% consumption of the introducedoxygen) does not become prohibitively short, i.e. less than 10 minutes.The "turn around" time required at the end of each cycle for venting thewaste gas, recharging the aeration chamber with oxygen gas, andrestarting the mixer is normally 2-3 minutes. Accordingly, if the mixingtime decreases below 10 minutes the "turn around" period of non-mixingbecomes an excessively large and ineffective portion of the total cycletime, despite the lower cost of low purity oxygen. Sufficient oxygenfeed gas is introduced to provide oxygen partial pressure of at least7.3 p.s.i.a., so that the gas-liquor mixing will be performed at leastat atmospheric pressure. Sub-atmospheric pressure are to be avoidedbecause of the resulting increased "turn-around" period, lower oxygentransfer rate, and possible atmospheric inleak to the oxygenationchamber.

For purposes of this description, the BOD-containing water and biomassin enclosure 11 will be referred to as "liquor." Means are provided formixing oxygen gas and liquor within oxygenation chamber 20, as forexample rotating surface-type impeller 30 powered by electric motor 31.The latter two components are joined by a shaft suitably sealed bycollar 32 against gas leakage through a central opening in the chambercover 22.

Discharge conduit 33 communicates with the upper portion of oxygenationchamber 20 for release of unconsumed oxygen-containing (spent) gas. Ventvalve 34 is provided in discharge conduit 33 and second pressure switch35 may be positioned therein upstream of valve 34.

The practice of a method aspect of the invention will now be describedusing the FIG. 1 apparatus. Assuming that the cycle begins with theoxygenation chamber 20 substantially completely filled with liquor andwith the mixer 30 de-energized, vent valve 34 is closed and shut-offvalve 28 is opened by the cycle control means. In this manner, firstfeed gas comprising at least 50% oxygen (by volume) and having oxygenpartial pressure of at least 7.3 p.s.i.a., is introduced to chamber 20and thereby progressively downwardly displaces a portion of the liquorfrom the upper end thereof into the liquor enclosure 11. Pressure switch29 senses the gradually rising feed gas pressure in conduit 26 and whena predetermined value is reached, a signal is transmitted through means36 to energize motor 31 and initiate mixing of gas and liquor.Alternatively the mixer 30 may be continuously operated during even thegas discharge step of each cycle, and in this event first pressureswitch 29 is not required. The advantage of including mixer energizingand deenergizing components in the cycle control means in that energyfor operating the mixer is consumed only when mixing is needed.

Gas inlet flow control valve 27 comprises one component of the system'sgas flow control means, and is set to maintain the oxygenation chambergas pressure at a predetermined level. This pressure is aboveatmospheric and preferably 15-25 p.s.i.a. If first pressure switch 29 isemployed, this predetermined level may be the same as or greater thanthat at which mixer motor 31 is energized. During the mixing step ofeach cycle, only sufficient oxygen gas is introduced through conduit 26to replace the net mass transfer of gas into the liquor. As nitrogen andcarbon dioxide impurities evolve from the mixed liquor into the overheadgas space, the oxygen content gradually declines. The mixing step ofeach cycle continues for at least 10 minutes and with sufficient mixingand gas-liquor contact energy to consume at least 60% (by volume) of theoxygen in the feed gas. As used in the description of this embodiment"feed gas" comprises the gas initially introduced to reach thepredetermined pressure plus any gas introduced during mixing to maintainthis pressure.

It should be recognized that the manner in which power is usedrepresents an important part of this invention. Two functions must beprovided: The solids must be held in suspension in the liquor (tomaintain the liquor-state), and the oxygen gas and liquor must becontacted intimately. In many previous aeration systems using air, thetwo functions were served by the air alone. The air volume was large, asrequired to provide the necessary oxygen from a gas highly diluted withnitrogen, and the stirring action of the gas on the liquor, whileinefficient, was adequate to hold the solids in suspension.

In this efficient oxygen aeration system, the quantity of gas needed tosupply the oxygen is much smaller and does not provide the mixing actionneeded to suspend the solids, especially when solids loadings are high.The energy needed for stirring the liquor is preferably supplied by amechanical agitator or propeller, which is considerably more efficientin this respect than gas bubbling. The stirrer may be a different devicefrom the gas-liquor contactor, as for example, a submerged propeller inconjunction with an additional gas sparger. Optionally, the twofunctions may be served by the same device but, in either event, thedevice should be of a type which sustains a high oxygen partial pressuredifferentiaL across the gas-liquor interfacial area generated by thedevice.

The energy required for gas-liquor contact is substantially less thanthe energy required by the solid-liquid suspension. The gas-liquorcontact energy is nevertheless significant, and unless the contactingmethod is properly chosen, the power consumption for this function canbecome excessive. Moreover, the DO (dissolved oxygen) level and oxygenutilization may suffer. A device should be chosen which generates alarge amount of interfacial area between gas and liquor, yet which doesnot produce a fine dispersion of liquid in the gas. Considerable poweris required to produce fine liquid dispersions and such dispersionsrepresent relatively inefficient mass transfer functions for thissystem. Suitable mixing devices produce a large contact area in a largevolume of liquor, such that the liquor adjacent the interfacial areadoes not approach saturation. The oxygen partial pressure driving forcesfor rapid solution are therefore sustained, and mixing losses arereduced when the liquor in the contacting zone returns to the main pool.Satisfactory mixing devices include spargers which produce fine bubblesin the liquid pool and surface aerators which throw relatively massivesheets or streams of liquid into the gas. Suitable devices are commonlycharacterized by the so-called "air standard transfer efficiency." Thisperformance parameter relates the rate of oxygen dissolution per unit ofinput horsepower from atmospheric pressure air into zero DO tap water at20° C. Suitable devices would have an air standard transfer efficiencyof at least 1.5 lb. O₂ /H.P. hrs.

One feature of this method is a balance between the overall level ofoxygen consumption in the liquor, and the average oxygen energy transferefficiency to the liquor. Extremely high overall percentage oxygenabsorption may be realized by continuing the mixing until virtually allof the oxygen in the feed gas is consumed. However, the energy transferefficiency would become prohibitively low, with extremely highdissolution power and capital investment costs. It has been discoveredthat these opposing characteristics may be be balanced in a methodvastly superior to the prior art by continuing the gas-liquor mixturestep for at least 10 minutes but for sufficient duration to consume atleast 60% of the oxygen in the feed gas to form fist oxygenatedliquid-solid and first unconsumed oxygen-containing gas. The lattercomprises 10-70% oxygen, but is of lower oxygen purity than the feed gasand has oxygen partial pressure of at least 1.47 p.s.i.a. That is, ifthe gas discharged from the chamber at the end of the mixing stepcomprises only 10% oxygen, it will be discharged at least at atmosphericpressure. Sub-atmospheric pressure is to be avoided for the previouslyindicated reasons. In a preferred embodiment the feed gas comprises atleast 90% oxygen, mixing is continued for at least 20 minutes, at least75% of the oxygen is consumed and the unconsumed oxygen-containing gascomprises 40 to 60% oxygen.

It is preferred to introduce oxygen at cycle average feed rates of0.10-0.50 lb. moles per horsepower hour of the mechanical mixing andgas-liquor contact energy during each of the succeeding oxygenationcycles. Lower feed rates limit the rate of oxygen dissolution into theliquor and higher oxygen feed rates provide more oxygen than can beeffectively mixed by this level of energy input and nominal gas-liquorcontacting effectiveness.

It is also preferred to introduce oxygen at cycle average feed rates of0.08-2.0 cu. ft. per cu. ft. of liquor. Lower oxygen feed rates limitthe biochemical oxidation reaction rate and higher rates provide moreoxygen than can be dissolved in the liquor per unit time.

The cycle control means also simultaneously close feed gas shut-offvalve 28 and open gas vent valve 34 at the end of the gas-liquor mixingstep of each cycle. As previously indicated, the mixer motor 31 may alsobe deenergized through signal transmitting means 36 at this point in thecycle. These changes may be instigated by any of several well-knownprocess monitoring means illustrated schematically as controller 37. Byway of illustration, an automatic preset timer could be employed andjoined to valves 28 and 34 respectively by signal transmitting means 38and 39. A timer would be most suited for relatively stable processconditions and long cycles. It should be noted, however, that the setperiod of such a timer could be altered by suitable controls tocompensate for variations in the volume and strength of BOD-containingwater fed to the system. Other compensation can be made for changes inDO level of the mixed liquor, for example by increasing or decreasingthe power input to mixer motor 31 and altering its speed of mixerrotation.

If the process conditions or the oxygen demand fluctuate significantly,it may be necessary to provide means for continuously analyzing theoxygen purity of the gas in oxygenation chamber 20. The valve changeswould then be initiated by a signal from the analyzer (not illustrated)indicating that the oxygen purity within the aeration chamber 20 hasdeclined to a predetermined value. The preferred purity for this stepchange depends on the feed gas purity and the relative costs of oxygen,oxygenator investment and power. For oxygen feed gas purities of atleast 90 percent, the spent gas discharge step is preferably initiatedat purities of 40 to 60%.

The spent oxygenation gas is vented through conduit 33 under the forceof the unbalanced hydrostatic head which causes the chamber 20 liquorlevel to rise and displace the gas. For this reason, the chamber must befixedly positioned below the liquor level, i.e. the edge of weir 23 isabove chamber cover 22. The cycle control means senses the end of thegas discharge step, simultaneously closing vent valve 34 and openingshut-off valve 28. This may for example be accomplished by secondpressure switch 35 set to operate at the reduced hydrostatic head in theoxygenation chamber attained when the liquor level rises to near cover22. Signal transmitting means 40 joins second pressure switch 35 tocontroller 37. The latter in turn communicates respectively with gasinlet shut-off valve 28 and vent valve 34 through signal transmittingmeans 38 and 39.

The aforedescribed cycle sequence is thereafter repeated in at least asecond oxygenation cycle wherein a second feed gas quantity comprisingat least 50% oxygen and having oxygen partial pressure of at least 7.3p.s.i.a. is introduced to chamber 20 through conduit 26 for mixing withsecond BOD-containing water and second biomass. This liquor ispreferably composed at least in part by the first oxygenatedliquid-solid, i.e. the product liquor from the first cycle. The degreeto which the first cycle product liquor comprises the second cycle feedliquor depends on several factors, including the relative sizes of theoxygenation chamber 20 and liquor storage enclosure 11, the liquor andgas flow rates, the feed gas oxygen purity, the gas-liquor contactenergy and mixing time, and the desired effluent water BOD-level. Itshould be understood that under some circumstances it may be desirableto discharge the first oxygenated liquid-solid through conduit 14 at theend of the first oxygenation cycle, and thereafter introduce the secondBOD-containing water and second biomass through conduits 12 and 13,respectively, before introduction of the second feed gas through conduit26 to chamber 20. The second and succeeding oxygenation cycles areperformed in an analogous manner. This constitutes a batch-typetreatment of BOD-containing water according to the invention.

As still another embodiment, additional BOD-containing water and biomassmay be introduced to enclosure 11 during each of the first and secondoxygenation cycles.

The FIG. 2 embodiment differs from FIG. 1 in several respects. Liquorstorage enclosure 11 is a naturally occurring reservoir, as for examplea lagoon. The active biomass is sludge circulated within the lagoon bynatural flow and submerged propeller 30. Part of this sludge gravitysettles to the lagoon bottom and may be periodically removed therefromby dredging means. Depending on the relative positioning of theBOD-containing water feed conduit 12 and oxygenator 20, the feed liquidand sludge mixing may, and in fact usually does occur prior to contactwith the oxygen gas in oxygenator 20.

The FIG. 2 mixing means includes sparger 45 submerged in the liquorbeneath impeller 30. The oxygen-containing gas bubbles discharged fromsparger 45 are distributed through chamber 20 in intimate contact withthe liquor and rise to the surface where the unconsumed portiondisengages into the gas space along with the reaction product gases. Toprovide the necessary pressure driving force for continuous circulationof oxygen gas through chamber 20, the inlet of compressor or blower 46is positioned in as flow communication with the chamber gas space as byconduit 47, and the discharge thereof is directed through conduit 48 tosparger 45.

When a submerged propeller and sparger are employed as in FIG. 2, thesystem should be operated such that the local downward velocity ofliquor is not sufficient to sweep the dispersed gas bubbles below thechamber lower end and outwardly into the uncovered portion of the liquorstorage enclosure. The gas must be substantially confined within thechamber and recirculated. The local downward liquid velocity andrecirculated. The local downward liquid velocity should be less than theterminal velocity of bubbles produced by the sparger to insure that thebubbles will rise.

Although the oxygenation chamber 20 and other connected apparatus may befixedly positioned in a lagoon, the FIG. 2 embodiment is arranged tofloat therein and is supported by flotation collar 49. A separateclarifier is not employed and the purified water is discharged throughconduit 50 having control valve 51 therein.

The practice of the instant method will be described in connection withthe FIG. 2 apparatus, starting with the introduction of a fistoxygen-containing feed gas quantity through conduit 26 and flow controlvalve 27, with vent valve 34 in gas discharge conduit 33 being closed.The liquor level in oxygenator 20 is thereby depressed relative to cover22, and liquor is forced back into the surrounding lagoon 11. Propeller30 may be continuously operated during this gas charging period ifdesired, and the gas flow control means may be used to energizemotor-driven blower 46 for gas recirculation when the chamber is filledwith the first quantity of oxygen-containing feed gas such that theliquor level reaches desired level "H" relative to cover 22 of chamber20. This function may be performed by low liquor level sensing means asfor example probe 51a. Signal transmission means 52 joins probe 51a withcontroller 53, which in turn transmits a signal through means 54 toclose oxygen feed valve 27. Controller 53 simultaneously energizesblower 46 through signal transmission means 54a.

In this embodiment, oxygen gas is not continuously introduced to chamber20 as makeup for that consumed by the liquor, so that the quantity ofgas therein continuously decreases as well as its oxygen purity. The gasflow control system includes means for sensing the chamber gas content,as for example probe 55 for monitoring the oxygen gas purity. Uponreaching a low predetermined oxygen concentration, a signal istransmitted through means 56 to second controller 57 which in turninitiates a cycle change by deenergizing recirculation blower 46 throughsignal transmission means 58 and opening vent valve 34 in dischargeconduit 33 through signal transmission means 59. First unconsumedoxygen-containing gas is expelled by first oxygenated liquid-solid andunoxygenated liquor rising in the chamber 20 and when the level rises tonear chamber cover 22, float switch 60 is actuated as part of the gasflow control means and in turn transmits a signal through means 61 tocontroller 53. The latter in turn sends a signal through means 54 toreopen oxygen feed valve 27 and another signal through means 62 tosimultaneously close vent valve 34. The second aeration cycle is thusinitiated and oxygen gas flows into chamber 20 until terminated by asignal from low liquor level probe 51a, as previously described.

FIG. 3 illustrates another embodiment wherein gas recirculation blower46 is used to exhaust the unconsumed oxygen-containing gas fromoxygenation chamber 20 at the end of the gas-liquor mixing step of eachcycle. This feature allows greater flexibility in locating theoxygenation chamber 20 relative to the liquor storage enclosure 11, asthe rising liquor level is not relied on to force out the unconsumedoxygen-containing gas. The gas flow control system includes cyclecontroller 38 which may for example be a timer or a gas purity analyzerplus a timer. Controller 37 is joined to feed gas shut-off valve 28 bysignal transmitting means 38 and also joined to vent valve 34 (in gasdischarge conduit 33) by signal transmitting means 39. Gas dischargeconduit 33 branches from the discharge side of blower 46.

During the gas discharge step valve 63 in the oxygenation gas recycleconduit 48 is closed, and the unconsumed oxygen-containing gas iswithdrawn by blower 46 for release to the atmosphere through opened ventvalve 34 in conduit 33. At the completion of this step, cycle controller37 closes vent valve 34 through means 39 while simultaneously openingfeed gas shut-off valve 28 through means 38 and oxygenation gas recyclevalve 63 through means 64. A predetermined gas pressure may then bemaintained in chamber 20 during the gas-liquor mixing step by gas inletflow control valve 27, as described in connection with FIG. 1.

It should be appreciated that whereas single oxygenators have beenillustrated and described in FIGS. 1-3 for simplicity, a plurality ofoxygenators may be positioned in a liquor storage reservoir and eitherconnected in parallel flow relation to the same oxygen gas source,individually connected thereto, or joined to different oxygen gassources. Moreover, all oxygenators may be operated identically or eachmay operate wholly independent of the others, e.g. on different cycletimes, different maximum purity levels in the gas before starting thespent gas discharge step, different gas recirculation rates (if blower46 is employed) and different power inputs to the various mixers.

Such flexibility of operation may be particularly advantageous where theliquor reservoir is a large basin or lagoon for waste, since the BODlevel will normally be higher near the waste inlet and consequently theoxygen demand will be greater in that zone.

FIG. 4 illustrates another multiple oxygenator arrangement wherein theliquor storage enclosure is a tank 11 having vertical partitions 65extending across the tank width from the bottom to above the liquorlevel. Partitions 65 together with the tank walls form a series ofcompartments joined by restricted flow openings 66 between adjacentcompartments. One or more oxygenation chambers 20 may be positioned ineach compartment with the rotating surface-type impeller 30 powered byelectric motor 31, as illustrated in FIG. 1. Oxygen gas is provided bymanifold conduit 67 joined to each of the multiple, e.g. fouroxygenation chambers through branch conduits 68. The latter contain gasinlet flow control valve 27, shut-off valve 28, and vent valve 34.Although not illustrated in the interest of simplicity, gas flow controlmeans as described in any of the single oxygenator chamber embodimentsof FIGS. 1-3 may be employed with modifications which will be apparentto those skilled in the art. Whereas the spent gas is discharged fromeach chamber 20 through its particular vent valve 34, the oxygenatedliquid-solid is flowed in stage-to-stage manner from the compartmentnearest BOD-containing water feed conduit 12 through restricted openings66 to the compartment from which the oxygenated liquid-solid isdischarged over weir 23 and through conduit 14. As the feed liquor for aparticular compartment comprises the oxygenated liquid-solid from theimmediately preceding compartment, the BOD content progressivelydecreases in the liquor from stage-to-stage.

The invention will be more fully understood by the following example inwhich apparatus similar to the FIG. 1 embodiment is used to cyclicallyoxygenate municipal-type waste liquor at 30° C. in a single stagetreatment with 99.5% oxygen gas at constant gas pressure. A singlesurface-type aerator of 100 horsepower rating is used and the "airstandard transfer efficiency" is assumed to be 3.00 lbs. O₂ /H.P.-hr. Aseparate clarifier is not employed but the active biomass is provided byforced circulation of activated sludge in the manner of FIG. 2. The"alpha" factor (ratio of aerator mass transfer efficiency in mixedliquor to that in pure tap water) was assumed to be 0.90, and the "beta"factor (ratio of equilibrium concentration of dissolved oxygen in mixedliquor to that in pure tap water) was assumed to be 0.95. The aerator ispositioned in a 30 feet diameter cylindrical chamber 4.0 feet deep(including 3.5 feet high gas space), the chamber volume being 2,830 cu.ft. The chamber is in turn positioned in a liquor storage enclosurewhich is also cylindrical and has the following dimensions; 65 feetdiameter, 20 feet deep, and 496,000 gallons liquor capacity.

Biochemical oxidation parameters for this example are listed in Table I,and are typical for a high rate activated sludge process treatingmunicipal sewage.

                  TABLE I                                                         ______________________________________                                        Feed, BOD        240 p.p.m.                                                   Effluent BOD      25 p.p.m.                                                   Feed flow rate    16 million gallons/day.                                     Oxygen consumption rate                                                                        200 p.p.m./hr.                                               Lb. O.sub.2 consumed/lb. BOD                                                  removed           0.70                                                        Feed liquid concentration:                                                     O.sub.2          0.0 p.p.m.                                                   N.sub.2          13.2 p.p.m.                                                  CO.sub.2         0.39 p.p.m.                                                 Mean oxygenation time for                                                     feed liquid       50 minutes.                                                 Total cycle time (oxygena-                                                    tion plus venting)                                                                              53 minutes.                                                 BOD loading      423 lbs. BOD/day 1000 ft..sup.3.                             ______________________________________                                    

FIGS. 5 and 6 show the relation of certain process variables andperformance parameters with time for 85% oxygen absorption and 50minutes cycle mixing time. In particular, FIG. 5 illustrates the gasphase mole fraction (left side ordinate) and liquor phase composition inp.p.m. (right side ordinate) for oxygen, nitrogen and carbon dioxide. Itwill be apparent that the oxygen gas partial pressure (Y_(O).sbsb.2)rapidly decreases during the first several minutes of oxygenation as theCO₂ content of the oxygenation gas rapidly increases (Y_(CO).sbsb.2).Fortunately, however, since CO₂ is about 35 times more soluble in theliquor (BOD-containing feed water and biomass) than is oxygen, theoxygenation gas rapidly reaches CO₂ equilibrium with the mixed liquor ata relatively low CO₂ partial pressure--a Y_(CO).sbsb.2 of about 0.14.This means that despite the continuous formation of additional CO₂ byvirtue of the biochemical reaction, the CO₂ concentration of theoxygenation gas remains virtually constant after the first few minutes,thereby minimizing its effect on the oxygen partial pressure. Incontrast to CO₂, the nitrogen content of the oxygenation gas(Y_(N).sbsb.2) gradually increases as the oxygen content decreases(Y_(O).sbsb.2), and the gas-liquor mixing step of the cycle isterminated when the oxygen partial pressure has declined to a level atwhich oxygen is no longer effectively consumed by the liquor. In thisexample, the Y_(O).sbsb.2 at the end of the 50 minute cycle is 0.58 or8.5 p.s.i.a. at atmospheric pressure. The unconsumed oxygen-containinggas is discharged in about 3 minutes to complete each cycle.

The liquor phase oxygen, nitrogen and carbon dioxide contents also varyconsiderably during each cycle. The DO content initially increases veryrapidly at the start of the cycle when the oxygen partial pressure(Y_(O).sbsb.2) and hence oxygen transfer rate to the liquor are high.The DO reaches a peak of about 5.9 p.p.m. after about 10 minutes andthen gradually decreases to approximately the initial DO level at theend of the cycle mixing step. The X_(N).sbsb.2 and X_(CO).sbsb.2contents vary in the reverse manner as X_(O).sbsb.2 since they are beingdesorbed from the liquid at initially very high rates (due to high rateof oxygen absorption) and then at necessarily lower rates during theremainder of the cycle. As this example represents periodic steady stateconditions, the liquor phase concentrations are equal at the beginningand end of the cycle as are the gas phase concentrations.

FIG. 6 shows the changes in instantaneous oxygen feed gas rate (curveA), instantaneous oxygen transfer rate to the liquor (curve B), and thecumulative overall percentage oxygen absorption throughout the cyclemixing step (curve C). The oxygen feed gas rate is initially zero andrapidly rises to a peak, then quickly decays to a gradually decreasingrate. The feed rate is initially zero at the start of the cycle due tothe extremely high CO₂ desorption rate (see Y_(CO).sbsb.2 of FIG. 5).This condition prevails for only about 30 seconds, however, and theoxygenation chamber pressure increase and liquor level change isnegligible.

Table II summarizes the pertinent oxygenation performance parameters andcompares them to air aeration performance capability at the same mixedliquor DO level.

                  TABLE II                                                        ______________________________________                                        Percent oxygen absorption    85                                               Length of cycle mixing step (min.)                                                                         50                                               Time average energy transfer efficiency (lbs. O.sub.2 /H.P.                   hr.)                         8.50                                             Time average liquor phase compositions (p.p.m.):                               Oxygen                      4.00                                              Nitrogen                    3.87                                              Carbon dioxide              198                                              Air operation energy transfer efficiency at same                              mixed-liquor conditions and DO level (lbs.                                    O.sub.2 /H.P. hr.)           1.48                                             Air operation energy transfer efficiency at same                              mixed liquor conditions and DO level of 2.0 p.p.m.                            (lbs. O.sub.2 /H.P. hr.)     2.04                                             ______________________________________                                    

Table II shows that an overall oxygen absorption efficiency of at least85 percent can be obtained while maintaining about a five-fold increasein average energy transfer efficiency relative to air operation at thesame mixed liquor conditions. Relative to a DO level of 2.0 p.p.m. whichis characteristic of current air operation, a four-fold increase inefficiency is attainable. It is thus apparent that the invention affordssubstantially higher oxygen absorption and average energy transferefficiency than conventional air aeration of BOD-containing water, asfor example municipal waste.

The use of a multiplicity of individually-cycled oxygenators in a singlemixed-liquor enclosure would largely dampen out the liquor phaseconcentration fluctuations shown in FIG. 5 (X_(O).sbsb.2, X_(N).sbsb.2,and X_(CO).sbsb.2), and afford a more uniform liquor composition. Thetime average mixed-liquor compositions of Table II should be quitesimilar to the average liquor compositions obtained in a multipleoxygenator system. Accordingly, the time average energy transferefficiencies of Table II are also representative for multipleoxygenators.

Although certain embodiments have been described in detail, it will beappreciated that other embodiments are contemplated along withmodification of the disclosed features, as being within the scope of theinvention.

What is claimed is:
 1. A method for treatment of BOD-containing water bycyclic oxygenation in contact with biomass comprising: as a firstoxygenation cycle.Iadd., introducing a first liquor of BOD-containingwater and biomass into a chamber having a closed upper end and a lowerend in fluid communication with a surrounding liquid storage enclosureand substantially filling said chamber with said first liquor,introducing into said chamber .Iaddend..[.mixing said BOD-containingwater and biomass as liquor and.]. a first feed gas quantity comprisingat least 50% oxygen (by volume) and having oxygen partial pressure of atleast 7.3 p.s.i.a. .Iadd.to progressively downwardly displace said firstliquor from an upper level in said chamber to a lower chamber level,mixing said first feed gas and first liquor in said chamber.Iaddend.while simultaneously continuously recirculating one of suchfluids against the other .[.fluids in a chamber.]. .Iadd.fluid.Iaddend.for at least 10 minutes and with sufficient mixing andgas-liquor contact energy input to consume at least 60% (by volume) ofthe oxygen in said first feed gas to form .Iadd.a .Iaddend.firstoxygenated liquid-solid and first unconsumed oxygen containing gas oversaid first oxygenated liquid-solid comprising 10-70% oxygen but of loweroxygen purity than said first feed gas and having oxygen partialpressure of at least 1.47 p.s.i.a. .[.;.]..Iadd., .Iaddend.dischargingsaid first unconsumed oxygen-containing gas from said chamber .Iadd.byraising a hydrostatic head of a second liquor of second BOD-containingwater and second biomass into said chamber until said chamber issubstantially filled.Iaddend.; as a second oxygenation cycle, .[.mixingin.]. .Iadd.introducing into .Iaddend.said chamber a second feed gasquantity comprising at least 50% oxygen (by volume) and having oxygenpartial pressure of at least 7.3 p.s.i.a. .Iadd.to progressivelydownwardly displace said second liquor from an upper level in saidchamber to a lower chamber level, mixing said second feed gas and secondliquor in said chamber .Iaddend..[., second BOD-containing water andsecond biomass.]. while simultaneously continuously recirculating one ofsuch fluids against the other .[.fluids.]. .Iadd.fluid .Iaddend.for atleast 10 minutes and with sufficient mixing and gas-liquid contactenergy to consume at least 60% (by volume) of the oxygen in said secondfeed gas to form second oxygenated luqid-solid and second unconsumedoxygen-containing gas over said second oxygenated liquid-solidcomprising 10-70% oxygen but of lower oxygen purity than said secondfeed gas and having oxygen partial pressure of at least 1.47 p.s.i.a.,said second BOD-containing water and second biomass .Iadd.of said secondliquor .Iaddend.being composed at least in part of said first oxygenatedliquid-solid .[.;.]. .Iadd., .Iaddend.and discharging said secondunconsumed oxygen-containing gas from said chamber .Iadd.by raising ahydrostatic head of liquor of BOD-containing water and biomass in saidchamber until the chamber is substantially filled with said liquor..Iaddend.
 2. A method according to claim 1 wherein additionalBOD-containing water and biomass are introduced to said chamber duringeach of said first and second oxygenation cycles.
 3. A method accordingto claim 1 in which said biomass is concentrated from said oxygenatedliquid-solid and recycled in sufficient quantity to provide volatilesuspended solids content of at least 3000 p.p.m. in said firstoxygenation cycle.
 4. A method according to claim 1 in which .[.saidchamber is substantially filled with liquor on discharging of unconsumedoxygen-containing gas therefrom at completion of an oxygenation cycle,said first feed gas is thereafter introduced and progressivelydownwardly displaces the liquor from the chamber upper portion, and.].the mixing .Iadd.steps .Iaddend.of said first .Iadd.and second.Iaddend.oxygenation .[.cycle.]. .Iadd.cycles each .Iaddend.is initiatedwhen the liquor is displaced to a predetermined chamber lower level. 5.A method according to claim .[.4.]. .Iadd.1 .Iaddend.in which part of.Iadd.each of .Iaddend.said first .Iadd.and second .Iaddend.feed gas isintroduced during the mixing .Iadd.steps .Iaddend.of said first.Iadd.and second .Iaddend.oxygenation .[.cycle.]. .Iadd.cycles,respectively. .Iaddend.
 6. A method according to claim .[.4.]. .Iadd.1.Iaddend.in which part of .Iadd.each of .Iaddend.said first .Iadd.andsecond .Iaddend.feed gas is introduced during the mixing .Iadd.steps.Iaddend.of said first .Iadd.and second .Iaddend.oxygenation .[.cycle.]..Iadd.cycles, respectively, .Iaddend.to maintain constant gas pressureand volume in said chamber. .[.7. A method according to claim 4 in whichthe lower end of said chamber is in fluid communication with asurrounding liquor storage enclosure and said first unconsumedoxygen-containing gas is discharged from said chamber by a risinghydrostatic head of liquor in the chamber..].
 8. A method according toclaim 1 wherein gas disengaging from the liquor during the mixing iscontinuously recirculated and reintroduced to said liquor. A methodaccording to claim 1 in which the oxygen feed gas is mechanically mixedwith said liquor at average rate of 0.10-0.50 lb. moles O₂ perhorsepower hour of mixing and gas-liquor contact energy input.
 0. Amethod according to claim 1 in which the oxygen feed gas is mixed withsaid liquor at average rate of 0.08-2.0 cu. ft. O₂ per cu. ft. liquor.11. A method according to claim 1 wherein said feed gas comprises atleast 90% oxygen, gas-liquor mixing is for at least 20 minutes, at least75% of the oxygen is consumed and the unconsumed oxygen-containing gascomprises 40-60% oxygen.
 12. A method for cyclic treatment of sewage byoxygenation in contact with activated sludge comprising:(a) providingsaid sewage and activated sludge as liquor within and substantiallyfilling a chamber having a closed upper end and a lower end in fluidcommunication with a surrounding liquor storage enclosure; (b)introducing first feed gas comprising at least 50% oxygen (by volume) ofsaid chamber in sufficient quantity to provide oxygen partial pressureof at least 7.3 p.s.i.a. and progressively downwardly displace theliquor from the chamber upper portion to a predetermined chamber lowerlevel; (c) as a first oxygenation cycle, mechanically mixing said firstfeed gas, sewage and activated sludge for at least 10 minutes whilesimultaneously continuously recirculating one of such fluids against theother fluids and also simultaneously introducing only sufficientadditional first feed gas to maintain constant gas pressure in saidchamber to form first oxygenated liquid-solid and first unconsumedoxygen-containing gas comprising 10-70% oxygen but of lower oxygenpurity than said first feed gas and having oxygen partial pressure of atleast 1.47 p.s.i.a., the mixing and gas-liquid contact energy inputbeing sufficient to consume at least 60% (by volume) of the oxygen insaid first feed gas; (d) discharging said first unconsumedoxygen-containing gas from said chamber by a rising hydrostatic head ofliquor in the chamber until the latter is substantially filled withliquor; and (e) thereafter consecutively repeating steps (b), (c) and(d) as subsequent oxygenation cycles.
 13. Apparatus for cyclicoxygenation of BOD-containing water comprising:(a) a liquor storageenclosure; (b) an oxygen gas source; (c) an oxygenation chamber having awall extending below the liquor level within said storage enclosure andits lower end in fluid communication with the enclosure, and a gas-tightcover; (d) oxygen supply conduit means between said oxygen gas sourceand said oxygenation chamber; (e) means for mechanically mixing saidoxygen gas and said liquor in said oxygenation chamber; (f) conduitmeans for discharging unconsumed oxygen-containing gas from the upperportion of said oxygenation chamber and having a vent valve therein; and(g) gas flow control means comprising: a gas inlet flow control valvearranged to maintain a predetermined gas pressure in said oxygenationchamber, and a shut-off valve in said oxygen supply conduit; means forsensing gas pressure in said oxygenation chamber; signal transmittingmeans from the pressure sensing means to said gas inlet flow controlvalve; and cycle control means for simultaneously closing said shut-offvalve and opening the gas vent valve, and thereafter simultaneouslyclosing said gas vent valve and opening said shut-off valve. 14.Apparatus for cyclic oxygenation of BOD-containing liquor comprising:(a)a liquor storage enclosure; (b) an oxygen gas source; (c) an oxygenationchamber within said storage enclosure having its lower end in fluidcommunication with the enclosure, and a gas-tight cover; (d) oxygensupply conduit means between said oxygen gas source and said oxygenationchamber and having a gas inlet flow control valve therein; (e) an oxygengas sparger positioned below the liquor level in said oxygenationchamber; (f) gas-liquor mechanical mixing means positioned below theliquor level in said oxygenation chamber; (g) a blower with the suctionside in flow communication with the oxygenation chamber upper portionand the discharge side in flow communication with said sparger; (h)conduit means for discharging unconsumed oxygen-containing gas from theoxygenation chamber upper portion and having a vent valve therein; and(i) gas flow control means comprising: low liquor level sensing meansassociated with said oxygenation chamber; signal transmitting means fromsuch low level sensing means arranged to close said gas inlet controlvalve when inflowing oxygen gas has downwardly forced the liquor levelto a predetermined elevation; means for sensing the chamber gas content;signal transmitting means to open said vent valve when the sensed gascontent descends to a predetermined value; high liquor level sensingmeans; signal transmitting means from such high level sensing meansarranged to close said vent valve and open said gas inlet control valvewhen the rising liquor reaches a predetermined elevation. .Iadd.
 15. Amethod for treatment of BOD-containing water by cyclic oxygenation incontact with biomass comprising: as a first oxygenation cycle,introducing a first liquor of BOD-containing water and biomass into achamber having a closed upper end and a lower end in fluid common with asurrounding liquid storage enclosure and substantially filling saidchamber with said first liquor, introducing into said chamber a firstfeed gas quantity comprising at least 50% oxygen (by volume) and havingoxygen partial pressure of at least 7.3 p.s.i.a. to progressivelydownwardly displace said first liquor from an upper level in saidchamber to a lower chamber level, mixing said first feed gas and firstliquor in said chamber while simultaneously continuously recirculatingone of such fluids against the other fluid for at least 10 minutes andwith sufficient mixing and gas-liquor contact energy input to consume atleast 60% (by volume) of the oxygen in said first feed gas to form afirst oxygenated liquid-solid and first unconsumed oxygen containing gasover said first oxygenated liquid-solid comprising a 10-70% oxygen butof lower oxygen purity than said first feed gas and having oxygenpartial pressure of at least 1.47 p.s.i.a., exhausting said firstunconsumed oxygen-containing gas from said chamber and raising ahydrostatic head of a second liquor of second BOD-containing water andsecond biomass into said chamber until said chamber is substantiallyfilled with said second liquor; as a second oxygenation cycle,introducing into said chamber a second feed gas quantity comprising atleast 50% oxygen (by volume) and having oxygen partial pressure of atleast 7.3 p.s.i.a. to progressively downwardly displace said secondliquor from an upper level in said chamber to a lower chamber level,mixing said second feed gas and second liquor in said chamber whilesimultaneously continuously recirculating one of such fluids against theother fluid for at least 10 minutes and with sufficient mixing andgas-liquid contact energy to consume at least 60% (by volume) of theoxygen in said second feed gas to form second oxygenated liquid-solidand second unconsumed oxygen-containing gas over said second oxygenatedliquid-solid comprising 10-70% oxygen but of lower oxygen purity thansaid second feed gas and having oxygen partial pressure of at least 1.47p.s.i.a., said second BOD-containing water and second biomass of saidsecond liquor being composed at least in part of said first oxygenatedliquid-solid, and exhausting said second unconsumed oxygen-containinggas from said chamber and raising a hydrostatic head of liquor ofBOD-containing water and biomass in said chamber until the chamber issubstantially filled with said liquor.